The EU is limited in which areas it is able to pass laws; these restrictions are set out in the Treaty on the Functioning of the European Union (TFEU), which you can read here: http://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:C2012/326/01&qid=1401280838965&from=EN — the relevant area begins at Title 1; Article 2.
The treaty establishes that “The Union shall have exclusive competence in the following areas:
- Customs Union
- Competition rules relating to the Internal Market
- Monetary policy for countries which have adopted the Euro
- Conservation of marine biological resources under the common fisheries policy
- Common commercial policy”
This means that only the EU may pass laws relating to these areas, although the third does not currently apply to the UK.
There are areas where the EU and member states have shared competence — this means that both the EU and member states may pass laws on these areas, but laws passed by member states may not contradict those passed at an EU level. These areas are:
- Internal Market
- Social Policy areas covered by the treaty
- Economic and Social cohesion
- Agriculture and fisheries, excluding conservation of marine biological resources
- Consumer protection
- Trans-European networks
- Area of freedom, justice and security
The final areas are those in which the EU may support, coordinate and supplement the actions of members, but cannot legislate. These are:
- Protection and improvement of human health
- Education, vocational training, youth and sport
- Civil protection
- Administrative cooperation
All other areas are assumed to be under the sole authority of the member states, hence areas such as the NHS and welfare fall under direct control of the UK government only — providing any changes do not contradict principles of EU law on things such as discrimination on grounds of nationality.